FR2515811A1 - INTERFEROMETRIC DEVICE FOR MAGNETIC FIELD MEASUREMENT AND ELECTRIC CURRENT SENSOR COMPRISING SUCH A DEVICE - Google Patents

Abstract

THE INVENTION RELATES TO THE MEASUREMENT OF MAGNETIC FIELDS. ITS OBJECT A MAGNETIC FIELD MEASUREMENT DEVICE INCLUDING A SOURCE OF LIGHT 1 POLARIZED STRAIGHTLY. A MAGNETOOPTICAL MEDIUM 4 IS ARRANGED ON THE PATH OF THE BEAM, THIS MEDIUM 4 BEING FOLLOWED BY A MIRROR 5 WHICH REFLECTS THIS BEAM. THIS MEDIUM 4 IS AN ENVIRONMENT IN WHICH THE PROPER MODES OF PROPAGATION ARE RIGHT AND LEFT POLARIZED, AND WHICH CONSERVES CIRCULAR POLARIZATION. A SEPARATOR ELEMENT BY POLARIZATION 2 IS PROVIDED BETWEEN SOURCE 1 AND MEDIUM 4, A DETECTION DEVICE 3 IS COUPLED. THE INVENTION APPLIES IN PARTICULAR TO THE MEASUREMENT OF ELECTRIC CURRENTS.

Description

INTERFEROMETRIC DEVICE FOR FIELD MEASUREMENT

  MAGNETIC AND ELECTRIC CURRENT SENSOR

COMPRISING SUCH A DEVICE

  The invention relates to a magnetic field sensor insensitive to

  external parameters such as temperature and pressure for example.

  In a ring interferometer, two beams travel in opposite directions on the same optical path, and interfere with the output of that path, provided that a disturbance of this path has the same characteristics for both directions of propagation and does not change during the transit time of the light in the interferometer, the two beams are assigned identically and their relative phase remains unchanged Disturbances of this type are called "reciprocal" Because the transit time in a 1 o interferometer is generally very small, the variations of a disturbance during this time, unless it is introduced voluntarily, are

generally negligible.

  But there are "non-reciprocal" disturbances that present a

  different amplitude in both directions of propagation, these are physiological effects

  which, by establishing its complete orientation, destroy the symmetry of

space or the middle.

  Two known effects have this property:

  the Sagnac effect, or relativistic inertial effect, where the rotation of

  ironometer with respect to a Gallilean landmark destroys the symmetry of the propagation times; the Faraday effect, or collinear magneto-optical effect, where a magnetic field creates a preferential orientation of the spin of the electrons of the

optical material.

  A device of the known art, described in the European patent application published January 28, 1981 under the number O 023 180 by the applicant, relates to a current measuring device comprising an optical fiber wound around a conductor in which circulates the current I to be measured, this optical fiber comprising one or more turns, the two ends of this wound optical fiber each receiving an optical wave coming for example from a laser; these two waves circulate in opposite directions in the fiber The current flowing in the conductor induces a magnetic field in the same direction as the direction of propagation of one of the waves and in opposite directions of the other The two waves emerging from the fiber have a phase shift A & which depends on the Verdet constant characteristic of the Faraday effect of the propagation medium, the intensity I of the current flowing in the conductor, possibly the number N of conductors when the optical fiber surrounds several conductor branches in which circulates the same current 1, and the number M of turns of the optical fiber surrounding the

driver.

  In order to highlight the phase shift between the two waves, this measurement device uses a "Sagnac" type interferometer structure, the two counter-rotating waves emerging from the ends of the fiber being recombined and the corresponding signal being detected by a photodetector. Thus these two waves undergo in the same way the reciprocal effects inducing in the medium variations, in the conditions of propagation, varying in the same direction, and undergo by the effect Faraday non-reciprocal of the variations in opposite directions. These variations in sense

  contrary are likely to be detected by an inter-

férométrique.

  Compared with this device of the prior art, the device of the invention has various advantages such as its great simplicity, its small number of components Moreover, no alignment is necessary, except that of the source and fiber, which is part of the known art This device also has a great geometric flexibility particularly as regards the waveguide length and in regard to the arrangement

  geometry given to this waveguide.

  In the device of the invention, only "non-reciprocal" disturbances

  proques "have an effect on the detected signal Dimensional variations

  such as creep, thermal expansion, pressure variation, o variances

  do not have any effect on the detected signal. In principle, therefore, there is a measuring instrument

  reciprocal "i which has perfect stability.

  In practice, for reciprocal disturbances to have an effect

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  rigorously zero, it is necessary that the two beams of the interferometer travel exactly the same route More precisely, it is necessary that the

  two waves are two identical solutions of the wave equation of the inter-

  féromètre, the sign of the parameter "temps" being reversed.

  When the interferometer is realized in free propagation, and this is the case of the use of discrete optical elements, this condition is never strictly respected: the wave equation presents a "continuum" of solutions and the less misalignment of the optics leads to the obtaining of different solutions, therefore of non-superimposed wave fronts; even for identical solutions when extension waves

  are considered, for example plane waves, the distribution of inten-

  sity, necessarily limited in practice, differs, in fact,

  of diffraction, and breaks the reciprocity.

  A monomode type solution therefore consists of a device realized

  in end-to-end waveguide structure.

  The subject of the invention is an interferometric device for measuring a magnetic field comprising a source delivering a rectilinearly polarized light beam, a propagation medium positioned in the path of the beam, a polarization separator element arranged between this source and this medium, a detection device coupled to this polarization separator element, characterized in that, the propagation medium being a magnetooptical medium, it comprises a mirror arranged perpendicular to the

  direction of propagation of this beam after crossing this medium.

  The invention also relates to an electric current sensor

having such a device.

  The invention will be better understood and other features will appear.

  will be with the aid of the description which follows with reference to the appended figures:

  Figure 1 shows the device of the invention; Figures 2 and 3 show two particular aspects of the device

of the invention.

  In the same way as in the case of free propagation in a material medium, the component of the magnetic field, collinear with the direction of propagation of the light in an optical fiber induces a non-reciprocal optical activity proportional to the intensity of the field

  Magnet and the Verdet constant of the material.

  This non-reciprocal optical activity, has its origin in the action of the magnetic field on the orientation of the spin of the electrons. This action results in a decomposition of the spectral lines into several symmetrical components compared to the original line (Zeeman effect). 'observation

  is made in the direction of the magnetic field, there are two

  Circumferential polarization whose spacing is proportional to the intensity of the magnetic field, the longer wavelength line generally having a circular polarization in the opposite direction to the direction of the current creating the magnetic field. The initial dispersion curve is therefore replaced by two offset curves, one corresponding to the right circular vibrations, the other to the left circular vibrations to the wavelengths of use, generally much greater than the wavelength of the

  absorption band, therefore appears a circular birefringence proportion-

  the Faraday effect depends on the material under consideration (Verdet constant). If it is very weak for diamagnetic materials, it goes into the intensity of the magnetic field and is related to its orientation and that of the light propagation, thus non-reciprocal. increasing successively for paramagnetic and then ferromagnetic materials It is important to note that only for diamagnetic materials, the Verdet constant

  is independent of the temperature Silica, material of which are constituted

  killed the low attenuation optical fibers, part of this class, an ammeter sensor made from such fibers benefits from this

  interesting property for the user.

  In the case of free propagatidn in a passive medium and in the case of guided propagation in an ideal monomode optical fiber, it is

  say in which two polarization modes polarized almost linear-

  identical intensity distributions and orthogonal polarizations

  (H Eil solution) are perfectly degenerate, in the absence of a magnetic field.

  the left and right polarizations see the same refractive index.

  This is no longer the case in the presence of a magnetic field, where the right and left polarizations see a different index It follows that a polarization

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  linear incident rotates at an angle a L proportional to the magnetic field H. It is important to differentiate the properties of magnetic circular birefringence (Faraday effect) from those of natural circular birefringence (natural rotation or optical activity). The Faraday effect depends on the direction of propagation of the light and is non-reciprocal, while the natural rotatory power is independent of the propagative direction of light and is reciprocal (a material is dextrorotatory or levorotatory regardless of the direction of light propagation It follows that if the light is reflected back through the material after reflection on a mirror, the rotation is doubled in the first case, whereas it is

canceled in the second.

  Optical fiber is sensitive to variations in the environment that

  can in particular induce reciprocal optical activity (mainly

  because of the elasto-optical interactions in torsion phenomena) which may evolve over time and which could naturally

  be perceived as a circular magnetic birefingence in a

  measurement method based on the determination of the angle t L after a single

crossing the optical fiber.

  In order to overcome this problem, it is important to use a configuration

  experimental device that is insensitive to the interfering effects of

  The configuration of the invention is particularly well suited.

  The invention proposed here shown in FIG. 1 makes use of the use of the magnetic effect in a guiding medium 4 such as a fiber

monomode optics for example.

  This effect corresponds to a non-reciprocal phase shift between two circularly polarized waves (the one right and the other left) under the action of the

  component of the magnetic field parallel to the direction of propagation.

  This phase difference can be expressed in the form:

AL = AVHL

  o V is the Verdet constant of the material, H the magnetic field applied parallel to the axis of the fiber and L the interaction length A

  is a constant depending in particular on the wavelength used.

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  To highlight the phase difference between the two waves, the measuring device according to the invention uses an interferometer structure, the two waves emerging from the medium 4 being recombined.

  and the corresponding signal being detected by a photodetector 3.

  This interferometer structure is a "Sagnac" type structure

  in which one would have applati the loop of measurement.

  If a light wave propagates in the middle 4, the magnetic field

  induced by magnetooptical effect, variations in the propagation conditions of the light wave When the magnetic field and the

  direction of propagation of the light wave are parallel, the magnetic effect

  tooptic induced on the light wave is a non-reciprocal effect, Faraday effect, depending on the direction of propagation of the wave compared to the direction of the

magnetic field.

  To highlight by this non-reciprocal effect a measurable quantity which is directly related to the magnetic field, and which by

  therefore does not integrate other effects and in particular the effects of

  also producing variations in the conditions of propagation

  (eg temperature variation or pressure variation), the

  device according to the invention uses two waves.

  These two waves undergo in the same way the reciprocal effects inducing in the medium variations in the conditions of propagation varying in the same direction, and undergo by the non-reciprocal Faraday effect variations in opposite directions. These variations in opposite directions are

  likely to be detected by an interferometric method.

  Indeed, linearly polarized light from laser 1 after having

  through the polarization separating prism 2 reaches the magneto-

  optical 4 whose own modes of polarization are polarized right and left These two polarized waves circularly right and left reach the mirror 5 The circularly polarized wave right after reflection on this mirror becomes circularly polarized on the left, and reciprocally the circularly polarized wave on the left becomes circularly polarized right

as shown in Figure 2.

  Thus this mirror 5 causes a change of direction and a change

  circular polarization of the waves that reach it.

  This medium 4 is made with any medium showing the Faraday effect, this effect being all the stronger as the material is more magnetic. With a ferromagnetic material this effect is greater than with a diamagnetic material. but in the latter case, this effect does not depend on the temperature. The light waves of left and right circular polarizations propagating in the medium 4 undergo a phase shift induced by the Faraday effect by the magnetic field. The magnetooptical medium induces polarization variations on the light waves that propagate. These effects are reciprocal and act on both. waves in the same way; against the magnetic rotation polarizing effect acting only on the circular polarization waves to introduce an advance or a delay on each of the components, acts in different directions on the two waves and thus introduces between them a global phase shift AI Indeed for these waves the establishment of the magnetic field parallel to the direction of propagation advances the circular vibration in the same direction as the magnetizing current

  and delays by an equal amount the circular vibration in the opposite direction.

  Even if the state of polarization of the light wave varies during its propagation along the fiber, the effects of advance and delay accumulate along this fiber and the two emergent waves have a phase shift likely to to be detected by interferometry directly representing the effects induced by the magnetic field, the other effects being experienced in the same way by the two waves and thus not introducing between them

of phase shift.

  The measuring device represented on the detector provides a luminous intensity I which varies as a function of the phase shift Ah between the two waves and therefore of the magnetic field, as represented on the curve at the

figure 3.

  If we consider the schematic diagram of Figure 1 The linearly polarized light from a laser 1 passes through a prism first

  polarization separator 2 It then passes through the magneto-

  optical 4 which will be assumed that the natural modes of propagation are polarized right and left After crossing this medium 4, these waves are

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  reflected on a mirror 5 and go back to the middle 4 to be analyzed

  by 2 The detection is ensured by a photodetector 3.

  The waves passing through the middle 4 from left to right have phases at the output: cc f nl L (right polarized wave) G cc n GL (left polarized wave) If we apply a magnetic field H: we induce phase shifts AMH and MH As shown in FIG. 2, at reflection on the mirror 5, the right wave becomes left. In the same way, the left wave becomes straight. The relative direction of the field and the direction of These waves emerge again from medium 4 with the phases: IDG "n DL + n GL + AH + ADH DGD 1 n GL + n DL MH ADH from o h D =" DG "GD = 4 m H and the phase shifts due to the natural birefringence of the medium and to

  reciprocal variations have disappeared.

  This phase shift AI) induces an appearance of light signal on the photodetector 3 reflecting the action of the magnetic field H. The sensor is therefore sensitive to the magnetic field and insensitive to

reciprocal parameters of the medium.

  Element 2 may be a polarization separator cube consisting of two glued prisms, the separation surface constituted by the hypotenuse of

  these two prisms being treated so as to be polarization splitter.

  This element has a preferential optical axis The incident rays having a direction of polarization parallel to this axis are transmitted in their entirety without modification, parallel to the direction of incidence. The incident rays having a direction of polarization orthogonal to the

  previous direction are fully reflected in an ortho-

  gonal to the direction of incidence The faces of the cube have also undergone a

  surface treatment to avoid parasitic reflections.

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  This element 2 could also be an integrated optical circuit powered by the source 1 directly coupled by the wafer to the circuit board

  integrated, in the same way as the medium 4 and the detector 3.

  After reflection on the mirror the two waves of circular polarizations follow the reverse optical path and are always circularly polarized. But the composite beam transmitted to the polarizer cube is again polarized linearly Indeed, as has just been explained, the output of the medium, the return of the two waves, the phase shifts due to birefringence

  natural environment and reciprocal variations have disappeared.

  In the middle 4 the rotatory power will compensate itself between the go and the

  return, unlike the Faraday effect that will accumulate.

  Thus, without an electric field and therefore without a Faraday effect, we obtain on the polarization separator the same linear polarization as in

  the go, and the resulting wave is therefore directed to the source.

  On the other hand, if there is a collinear magnetic field with the direction of the light wave, the projection of the resulting linear wave on the polarization at ir / 2 is no longer zero, and a part of the light wave dependent of the magnetic field will reach the detector, indeed at the crossing of the polarization separator cube it will be reflected by the

common face of the cube.

  To make the medium 4, we can consider an optical fiber having little or no linear birefringence This fiber can be made of a material having a more pronounced Faraday effect such as iron garnet and Itrium (Yig) It can also be made of silica for which the effect

  Faraday is weak but independent of temperature.

  Single-mode fibers normally manufactured for telecom-

  still have a small amount of linear birefringence

  area, and circular birefringence As a result these fibers do not retain the

  linear polarization, nor the circular polarization.

  It is possible to make the fiber very birefringent linear by breaking

  circular symmetry in favor of a planar symmetry.

  It is also possible to consider an inverse method, which consists in introducing a circular birefringence or rotary power,

  raised so as to maintain the circular polarization.

  One solution for creating this circular polarization is to

  to put the fiberglass to a torsional stress, for example applied externally by twisting between its two ends: an effect of the twisting of this fiber is to introduce a circular birefringence therein Various methods of the prior art allow then to keep this

  state of torsion It can be, in particular, using a mechanical hoop.

  The sensor can advantageously be made using a single-mode fiber retaining the circular polarization, the end of which has been metallized (or treated) so as to be totally reflective. The laser used may be a semiconductor laser. The optical fiber retains polarization. circular can be made by twisting a single-mode fiber with a rate of the order of a few tens of turns per meter This fiber can be either left free or wrapped around a conductor for

make a current sensor.

  One can consider a fiber for which each of the elements of diffusion; impurity or discontinuity of this fiber, constitutes a reflective element. If we send a light pulse, generated by a pulsed laser for example, we can then perform a temporal analysis of these multiple reflections, which allows a longitudinal exploration of this fiber This backscattering method allows a measurement of the field

magnetic along the fiber.

  We can also operate in the opposite way, considering the magnetic field as known, and study the properties of the diffusing elements.

  in the fiber to thus characterize it.

  It is obvious that any conventional method of phase modulation allowing to work around a zero described for other sensors of

  the known art is applicable for this device of the invention.

  This device of the invention can be used to make a current sensor by making a loop of one or more turns using this fiber around the conductor in which this current flows. magnetic field parallel to the direction of propagation advances the circular vibration in the same direction as the magnetizing current and delays an equal amount the circular vibration in the sense

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  contrary Even if the state of polarization of the light wave varies during its propagation along the fiber, the effects of advance and delay accumulate along this fiber and the two emergent waves present a

  phase shift that can be detected by interferometry

  the effects induced by the current I flowing in the conductor, the other effects being experienced in the same way by the two waves and

  thus not introducing phase shift between them.

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Claims (9)

  1 Interferometric device for measuring magnetic field   carrying a source (1) delivering a polarized light beam rectified   linearly, a propagation medium (4) positioned in the path of this beam, a polarization separator element (2) disposed between this source (1) and this medium (4), a detection device (3) coupled to this element polarization separator (2), characterized in that, the propagation medium (4) being a magnetooptical medium, it comprises a mirror (5)   arranged perpendicular to the direction of propagation.   2 Device according to claim 1, characterized in that the magnetooptical medium (4) has its own polarized propagation modes right and   left, this medium retaining the circular polarization.   Device according to Claim 1, characterized in that the source (1) is a semiconductor laser.   4 Device according to Claim 1, characterized in that the element   The polarization separator (2) comprises two refractive prisms contiguous to their hypotenes so as to form a semi-reflecting separating surface. Device according to Claim 1, characterized in that the element   The polarization splitter is an integrated optical element.   6 Apparatus according to claim 1, characterized in that the propagation medium (4) is a monomode fiber retaining the circular polarization. Device according to claim 6, characterized in that the fiber is twisted.   8 Apparatus according to claim 6, characterized in that the fiber is in silica.   Apparatus according to Claim 6, characterized in that the fiber is made of iron garnet and Itrium.   Device according to Claim 6, characterized in that the end the fiber is metallized.   11 Apparatus according to claim 1, characterized in that the   source (1) is a pulsed laser.   12 Electrical current sensor Faraday effect, characterized in that   it comprises a device according to any one of claims I to 11,   the optical fiber forming at least one loop around a conductor in which current flows.